In recent years, energy conservation has been considered as a significant technical objective even in the fields of electrophotographic apparatuses such as copiers and printers. In order to achieve the objective, considerable reduction of the amount of heat used in fixing devices has been demanded. For this reason, there has been a demand for toners that are fixable with a small amount of heat, that is, toners having a good low-temperature fixability.
In addition, there has been a strong need for high-speed printing. When continuous printing for a large number of sheets is performed at a high speed, heavy load is applied to the toner due to the toner being rubbed against members constituting a development device and the toner tends to become degraded. The degraded toner causes contamination of the members, which may cause degradation of chargeability or image defects such as development streaks. Accordingly, there has been a demand for a toner having enhanced stress resistance to suppress degradation of the toner.
In order to provide a toner having both low-temperature fixability and stress resistance, the toner desirably has the following characteristics: the toner easily deforms under a pressure load during fixing, but has a sufficiently high strength against a relatively light pressure load.
An effective known method of achieving fixing at a lower temperature is to use a binder resin that contains a component having a crystalline structure.
Crystalline polyester has molecular chains that are regularly arranged and, as a result, does not clearly undergo glass transition, does not soften in a temperature range lower than the crystalline melting point, but melts at a temperature slightly higher than the melting point. Crystalline polyester thus has what is called a sharp melting characteristic.
In view of this feature, toners containing crystalline polyester as a component having a crystalline structure have been studied.
Patent Literature 1 proposes a toner including, as a binder resin, a block polymer including, in a crystalline segment, a crystalline polyester component or a crystalline polyurethane component and, in an amorphous segment, an amorphous polyester component or an amorphous polyurethane component.
Use of such a block polymer as a binder resin can impart the sharp melting characteristic to the toner. However, amorphous resin films (shells) formed in the surfaces tend to cause loss of the sharp melting characteristic of the toner on the whole. Thus, it was found that the low-temperature fixability needs to be improved.
Patent Literature 2 proposes a toner including, as a shell material, a crystalline resin prepared from a monomer including a long chain alkyl group or a crystalline polyester chain. However, when the crystalline resin is used as a shell material alone, the low-temperature fixability is not sufficiently improved and the low-temperature fixability needs to be improved.
Thus, in a toner having a core-shell structure, use of resins having a crystalline structure as both of the core material and the shell material was considered to improve the low-temperature fixability.
Patent Literature 3 proposes a toner including, as a core material, a block polymer including a crystalline segment and an amorphous segment bonded together and, as a shell material, a resin including a crystalline polyester chain.
According to this method, the toner has an enhanced sharp melting characteristic. However, the following has been found: the crystalline polyester does not necessarily have sufficient stress resistance; when the crystalline polyester segment is present in a large quantity in the toner surface, toner deformation is caused in a development device during continuous printing for a large number of sheets and image defects such as development streaks tend to be caused.
Patent Literature 4 proposes a toner including, as a core material, a resin containing a block polymer including a crystalline polyester and an amorphous polyester. In this literature, this block polymer is used as the shell material to produce the toner having a core-shell structure. It is alleged that press fixing of this toner is allowed by using plastic behavior of the polyester block polymer under the application of a certain pressure or more and the toner can have high toner strength in a development device. The inventors of the present invention produced this toner and evaluated fixability with a standard film-fixing fixing device. However, sufficient fixability was not achieved. Accordingly, such a toner still needs to be improved in terms of achieving both sharp melting characteristic and low-temperature fixability.
As described above, in order to achieve a toner having both low-temperature fixability and stress resistance, a toner to which a component having a crystalline structure has been introduced still needs to be improved.